Numerous processes are known for the separation of air by cryogenic distillation into its constituent components. Typically, the air separation process involves removal of contaminant materials such as carbon dioxide and water from a compressed air stream prior to cooling to near its dew point. The cooled air then is cryogenically distilled in an integrated multi-column distillation system.
Processes to produce a high purity nitrogen stream containing few light contaminants, such as hydrogen, helium and neon have been proposed. Concentration of some of these contaminants in the feed air can be as high as 20 ppm. Almost all of these light components show up in final nitrogen product from an air separation unit (ASU). In some cases, such as for the electronic industry, this contamination level is unacceptable in the end use of this nitrogen product.
The following patents disclose approaches to the problem.
U.S. Pat. No. 4,824,453 discloses a process for producing ultra high purity oxygen as well as high purity nitrogen, where the nitrogen purity exceeds 99.998% and the amount of impurities is generally less than 10 ppm. More specifically, air is compressed, cooled and distilled in a rectification system wherein in a first stage rectification an oxygen enriched fraction is removed from the bottom and a nitrogen rich liquid fraction is removed from an upper portion of the first stage rectification, sub-cooled and returned as reflux to the top of the second stage rectification. A nitrogen rich liquid is removed from an upper portion of the second stage at a point just below an overhead removal point for nitrogen vapor from the second stage rectification. Liquid oxygen from the bottom of the first stage is sub-cooled, expanded and used to drive a boiler/condenser in the top of the high purity argon column. Nitrogen vapor from the top of the first stage is used to drive a reboiler/condenser in the bottom of a high purity oxygen column. To enhance product purity, a portion of the gaseous nitrogen stream from the top of the first column is removed as purge.
U.S. Pat. No. 4,902,321 discloses a process for producing ultra high purity nitrogen in a multi-column system. Air is compressed, cooled and charged to a first column where it is separated into its own components generating an oxygen liquid at the bottom and a nitrogen rich vapor at the top. The oxygen liquid is expanded and used to drive a boiler/condenser which is thermally linked to the top of the first column for condensing the nitrogen rich vapor. A portion of the nitrogen rich vapor is removed from the top of the first column and condensed in the tube side of a heat exchanger. The resulting liquid nitrogen is expanded and charged to a top of a stripping column wherein nitrogen including impurities are flashed from the stripping column. Any impurities not removed by flashing are stripped by passing a stream of substantially pure nitrogen upwardly through the column. The nitrogen liquid collected at the bottom of the stripping column is pumped to the shell side of the heat exchanger, vaporized against the nitrogen-rich vapor and removed as high purity product.
European Patent 0 0376 465 discloses an air separation process for producing ultra high purity nitrogen product. In the process, nitrogen product from a conventional air separation process is charged to the bottom of a column equipped with a reflux condenser. Liquid nitrogen is withdrawn from an upper portion of the column and flashed generating a liquid and a vapor. The liquid obtained after flashing is then flashed a second time and the resulting liquid recovered.
There are essentially two problems associated with the processes described for producing ultra-high purity nitrogen and these problems relate to the fact that in the '453 disclosure purities are quite often not sufficiently high to meet industry specifications and in the '321 process nitrogen recoveries are too low. The same can be said of the '465 European patent.